1. Field of the Invention
The present invention relates to a magenta toner and a magenta developer for developing electrostatic images in electrophotography, electrostatic recording, and electrostatic printing.
2. Discussion of the Background
In electrophotographic image forming apparatuses and electrostatic recording apparatuses, electric or magnetic latent images are formed into visible images with toner. Specifically, in electrophotographic image forming apparatuses, an electrostatic latent image is formed on a photoreceptor and is developed with a toner to form a toner image. The toner image is transferred onto a transfer medium and is fixed thereon by application of heat, etc.
A typical toner for developing electrostatic latent images is comprised of colored particles comprising a binder resin and additives such as a colorant and a charge controlling agent. Methods of producing toner are broadly classified into pulverization methods and suspension polymerization methods.
In pulverization methods, raw materials such as a binder resin (typically a thermoplastic resin), a colorant, a charge controlling agent, an offset inhibitor, etc., are melt-kneaded. The melt-kneaded mixture is pulverized into particles, and the particles are classified by size to obtain a desired-size toner.
Pulverization methods generally provide toners having good properties. However, the range of choice for raw materials is narrow. For example, it is preferable that the melt-kneaded mixture can be pulverized using economical apparatuses. From this viewpoint, raw materials should be chosen so that the melt-kneaded mixture is made as brittle as possible.
Disadvantageously, such a brittle melt-kneaded mixture may be pulverized into particles with a wide size distribution. To obtain a toner which provides high-resolution and high-gradation images, ultrafine particles having a particle diameter of 5 μm or less, preferably 3 μm or less, and coarse particles having a particle diameter of 20 μm or more may be removed, for example, but this results in an extremely low yield.
Pulverization methods have another disadvantage that it is difficult to evenly disperse additives such as a colorant and a charge controlling agent in a binder resin. Therefore, the colorant may disadvantageously expose at the surface of a toner, degrading chargeability of the toner.
As just described, pulverization methods do not satisfactorily respond to recent demands for high-performance toner yet.
To overcome these disadvantages of pulverization methods, suspension polymerization methods have been proposed.
Suspension polymerization methods generally provide spherical toners. Disadvantageously, spherical toners are difficult to remove from the surface of photoreceptors.
When an image with a low toner image area ratio is transferred, only a slight amount of toner particles may remain on the photoreceptor, which may cause no problem. By comparison, when an image with a high toner image area ratio, such as a picture image, is transferred, a relatively large amount of toner particles may remain on the photoreceptor. In this case, the resultant image background may be contaminated with toner particles. This phenomenon is hereinafter referred to as “background fouling”. Additionally, charging members for charging the photoreceptor may be also contaminated with toner particles, degrading charging ability of the charging members.
In suspension polymerization methods, a polymerization reaction for producing a resin is performed simultaneously with production of toner particles. For this reason, most of the raw materials which are conventionally used for pulverization methods may not be directly applied to suspension polymerization methods. Even in a case in which conventionally-used raw materials are applied to suspension polymerization methods, the particle diameter of the resultant toner particles may not be controlled as desired under the influence of additives such as a resin and a colorant. It may be said that usable materials for suspension polymerization methods are limited.
Because polyester resins that provide excellent fixing and color properties cannot be used for suspension polymerization methods, suspension polymerization methods cannot contribute to downsizing, speeding-up, and colorization of image forming apparatuses. In attempting to solve this problem of suspension polymerization methods, Japanese Patent No. (hereinafter “JP”) 2537503 discloses a toner production method in which fine resin particles obtained by an emulsion polymerization are coalesced to form toner particles. This method produces irregular-shaped toner particles.
However, this method has a disadvantage that surfactants that are used in the emulsion polymerization may remain in large amounts both on the surface and inside of the toner particles even when the toner particles are subjected to washing with water. Therefore, the resultant toner may have poor chargeability and the resultant image background may be contaminated with toner particles. The remaining surfactants may also contaminate photoreceptors, charging members, and developing members. In addition, colorants may aggregate in the toner particles, which results in deterioration of chargeability of the toner.
In full-color image formation, slight deterioration in developability or transferability of toner may cause significant deterioration in color balance and gradation of the resultant image.
Generally, colorants are hydrophilic and incompatible with resins. Therefore, transmitted light is reflected diffusely at an interface between the colorant and the resin. Accordingly, colorants generally degrade transparency of toner, which results in low transmittance of an OHP (overhead projector) sheet when a toner image is formed thereon. When colorants are not finely dispersed in toner, transmittance of an OHP (overhead projector) sheet may be much lower.
Unexamined Japanese Patent Application Publication No. (hereinafter “JP-A”) discloses a toner production method which includes steps of dissolving or dispersing a pigment which is surface-treated with a fatty acid and a pigment dispersing agent in a first organic solvent which solubilizes a binder resin, to prepare a pigment dispersion; mixing the binder resin and the pigment dispersion with a second organic solvent to prepare an oily component; suspending the oily component in an aqueous medium to form fine particles; and removing the organic solvents from the resultant suspension to obtain toner particles. Although the pigment is surface-treated with a fatty acid, the fatty acid does not include an amino group that is capable of controlling chargeability of toner.
JP 3661422 discloses a toner which includes a polymer dispersant as a pigment dispersing agent. It is disclosed therein that the acid value and amine value of the polymer dispersant are specified so that the resultant toner has a good combination of offset resistance, chargeability, storage stability, and coloring property, and transparency. However, it may be said that storage stability is not satisfactory.
This toner further includes a synergist, which is a derivative of a pigment, as an auxiliary pigment dispersing agent. A synergist is produced by introducing a polar group to a pigment, and improves interactions between the pigment and pigment dispersing agents so that the pigment is finely dispersed in toner.
However, there is a problem that synergists allow pigments to migrate to the surface of the resultant toner or to an aqueous medium when the toner is produced in the aqueous medium. This may be because synergists have a polar group, as described above, and the polar group generally has hydrophilicity. Synergists adsorb to pigments while the polar group is hydrophilic. Therefore, pigments may migrate to the surface of the resultant toner or to an aqueous medium. In this case, the resultant toner may have poor coloring power and poor fixing property, and pigments are likely to contaminate other members.
Recently, toners for producing full-color images generally include a release agent to eliminate oil applicators from fixing devices. Such toners including a release agent are hereinafter referred to as “oil-less toners”. Release agents are more difficult to evenly disperse in toner compared to colorants. When release agents are unevenly dispersed in toner, chargeability, developability, storage stability, and transparency may be poor.
In the field of process printing, yellow, magenta, and cyan compose the three primary colors.
Pigments are widely used as colorants in various fields of image recording methods, such as conventional printing using plates, electrophotographic recording, ink-jet recording, and thermal transfer recording.
To more improve reproducibility of color in the above image recording methods, demands for vivid and transparent image recording agents of yellow, magenta, and cyan are increasing.
A document “Description of ISO/Japan Color Offset Sheet Printing Color Standard Japan Color Color-Reproduction Printing 2001 (The Japanese Society of Printing Science and Technology, Japan Printing Machinery Association, and Japanese domestic committee of International Standardization Organization print technology committee ISO/TC 130)” describes ISO/Japan Color and provides standard inks, standard papers, and standard colors.
Among the standard papers, art paper is the best at reproducing color. However, it is difficult for conventional electrophotographic magenta toners to reproduce coloring power, color saturation, and color hue even on art paper.
In particular, there has been no magenta pigment which can reproduce the standard magenta color on art paper that is standardized in Japan Color among pigments with low light stability and low hydrophilicity, which are usable pigments for toner production methods using aqueous medium.
Quinacridone pigments have been widely used as magenta pigments from the viewpoint of their color hue and light stability. However, the coloring power of quinacridone pigments is poor, and therefore a naphthol pigment C. I. Pigment Red 269 is also widely used. Since C. I. Pigment Red 269 is more reddish compared to the standard magenta color on art paper that is standardized in Japan Color, there have been attempts to use a naphthol pigment and a quinacridone pigment in combination, as disclosed in JP-A 2003-215847 and JP-A 2003-202706. However, because the backbone structures of naphthol pigments and quinacridone pigments are different, their combinations may degrade transparency and color saturation.
Because of these reasons, an electrophotographic magenta toner which can respond to recent demands for high-performance toner is still needed.